Hi Scallawalla,
The Cotton Muslin cover would be a more basic and price-conscious option. The Mattresses.net cover you linked to is quilted to wool and would be more substantial, offer better structural integrity, better UV/oxidation protection, and also provide some of the extra benefits of wool (comfort, temperature/humidity assistance).
You are correct. I wish there was a mattress that would be 100% guaranteed to take this pain away from you. I’d recommend it in an instant!
I’m a fan of pillows that you can shape and adjust the fill, and that really can assist with shoulder pain.
In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin, although there are other benefits between different types of mattress encasements (as I mentioned previously in this reply).
There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range. You can read more about phase change materials in post #9 here and at the end of post #4 here. Their results tend to be temporary.
Phoenix